Thermostats typically used in refrigerators have electrical contacts that "make" and "break" contact to complete an electrical circuit to energize the refrigerator compressor motor or power relay. There may be a small arc or spark produced by the thermostat contacts by this process. This is not a problem in refrigerators that use FREON or other non-explosive refrigeration gasses. However, some countries are presently moving in a direction to use propane, butane, or other hydrocarbon-based gas in lieu of FREON for a variety of reasons.
One such reason to eliminate the use of FREON gas is that FREON gas is a chloroflurocarbon-type gas (CFCs) and is known to be detrimental to the environment. Public pressure and governmental regulations have been increasing the demand on industry to use "environmentally-friendly" products. As a consequence, refrigeration appliances have been developed in which the refrigerant fluid is made up of gases or a mixture of gases that do not significantly impact the environment adversely. However, such relatively environmentally friendly gases, such as propane and butane, have the undesirable characteristic of being flammable.
In refrigeration systems using such combustible gases, some of the gas could be present in the closed food storage compartment where the thermostat assembly is located. This may pose a risk of explosion if the gas is ignited. Depending upon the propane or butane to oxygen ratio, a spark produced within the thermostat could cause ignition of the gases within the refrigeration compartment. Such a situation is dangerous and unacceptable. Additionally, various safety specifications, such as IEC 79-15, dictate that certain precautions must be taken and that certain design criteria must be followed when explosive gasses may be present.
In specifying temperature settings for vapor operated refrigeration thermostats, it is known to include a standard barometric pressure at which the specified temperatures apply. When performing actual temperature setting measurements, the actual barometric pressure is also measured, and the temperature settings are corrected to the specified standard barometric pressure.
When the thermostat mechanism is placed inside a sealed enclosure, measurement of the pressure inside the sealed enclosure is no longer possible. Thus, measurement of the setting temperatures now vary as a result of variation in atmospheric pressure at the time that the enclosure was sealed, and also vary as a result of measuring the temperature settings of the thermostat having a body temperature different than the body temperature at the time of sealing. The result is an apparent increase in setting temperature variation and different mean setting temperatures. This causes temperature setting measurement correlation problems between the thermostat manufacturer and the customer which incorporates the thermostat into a refrigeration unit.